The present invention relates to a system and method for use in the autotransfusion of blood. More particularly, the invention relates to an improved closed autotransfusion system which is automatically regulated by closed-loop feedback of relevant patient hemodynamic variables.
Autotransfusion involves the reinfusion of a patient's own blood as opposed to the transfusion of bank blood. Although intraoperative and postoperative blood conservation techniques have decreased the use of bank blood for cardiac surgical patients, the increasing number of surgical procedures has placed a strain on the bank blood supply. One procedure to reduce this strain is postoperative autotransfusion, the return of blood shed from the mediastinum or thoracic cavity following surgery. The efficacy and safety of postoperative autotransfusion has been documented (Johnson et al. Ann Thorac. Surg. 1983; 36:178). Furthermore, autotransfusion is the safest form of transfusion therapy because there is no risk of alloimmunization, hepatitis, acquired immunodeficiency syndrome (AIDS), or other complications possible with bank blood.
An autotransfusion system in which shed mediastinal blood is filtered and collected in a transfusion bag located within a canister is described in Schaff et al., J. Thorac. Cardiovasc. Surg. 1978; 75:632. When the volume of blood in the transfusion bag is adequate for reinfusion, the bag is removed from the canister, compressed to remove any air from the bag, and attached to a blood administration set. Blood collected within the bag is utilized for autologous blood infusion only if 400 ml or more is collected within a four hour period. This system, however, is inconvenient to use, costly, and has the potential for bacterial contamination. Furthermore, this system reinfuses autologous blood after approximately a four hour delay and thus does not respond quickly to hemodynamic instability caused by blood loss.
Cosgrove et al., Ann. Thorac. Surg. 1985; 40:519, describes a closed autotransfusion system in which the cardiotomy reservoir used during surgery is reconfigured to serve as a receptacle for postoperative mediastinal drainage. Blood drains through a chest tube and collects in the bottom of the cardiotomy reservoir after being filtered and then is reinfused using a standard infusion pump. Every hour, mediastinal drainage is measured and the infusion pump is manually adjusted to deliver this amount of blood during the next hour. Because this system reinfuses blood after an hour delay, the system does not allow immediate adjustment of the rate of infusion in response to patient hemodynamic instability nor does the system allow for immediate reinfusion of shed blood. Furthermore, the system requires that the nursing staff manually perform routine maintenance every hour the system is operational.
Cardiac index (heart rate times stroke volume per square meter of body surface area) is known to be an important indicator for predicting survival and recovery in the postsurgical cardiac patient. Heart rate can be paced postsurgically when necessary for optimal cardiac index but convenient methods for reliable, repetitive measurements of stroke volume in the clinical environment do not yet exist. However, the left atrial pressure (LAP) correlates with the stroke index (stroke volume per square meter) and thus is one of the determinants of stroke volume. Infusion of blood increases LAP and thus infusion can be used to affect a patient's hemodynamic stability. However, when LAP exceeds 12-15 mm Hg in most adult patients, the cardiac index no longer rises. Thus, 12-15 mm Hg is an optimal mean LAP for most adult patients.
Sheppard et al., Ann. Surg. 1968; 168:596 and Sheppard et al., Fed. Proc. 1974; 33:2327 utilize this concept in their cardiac surgical intensive care computer system. This system provides for the infusion of banked blood using closed-loop feedback based upon a combination of the patient's left atrial pressure (LAP) and the total blood infused-to-total blood drained ratio. A constant rate 20 ml dose of blood is automatically infused every two minutes so long as neither the LAP nor the total blood infused-to-drained ratio exceeds a preset limit; if these limits are exceeded, no blood is infused. Although this bank blood system attempts to stabilize a patient's hemodynamic characteristics by infusing blood to raise a patient's LAP, this system is incapable of responding minute-by-minute to a patient's blood loss; blood loss is measured only to prevent overinfusion. Furthermore, in the event of a computer-to-pump communication failure or a general computer failure during infusion, the system continues to infuse bank blood at a constant rate regardless of the patient's LAP or total blood infused-to-blood drained ratio. No provision is made to keep a patient's vein open during periods that either the LAP or the total blood infused-to-drained values exceed their limits. Also, the LAP and chest drainage measurements are susceptible to infusion-related distortions and the patient does not receive the benefits inherent with autologous blood.
Therefore, there exists a need for a fully-automated, closed, closed-loop autotransfusion system that employs an adjustable infusion rate to maintain a patient's total blood volume and respond minute-by-minute to a patient's hemodynamic instability. As used herein, the term "closed system" refers to a system that is protected from bacterial contamination; the term "closed-loop system" refers to a system in which the imput or excitation to the system is automatically changed to produce a desired output.